7-[4-(4-Benzo[b]thiophen-4-yl-piperazin-1-yl)butoxy]-1H-quinolin-2-one (hereunder referred to as Compound (I)) or a salt thereof is known to act as a dopamine D2 receptor partial agonist, a serotonin 5-HT2A receptor antagonist, and an α1 adrenergic receptor antagonist, as well as a serotonin uptake inhibitor (or a serotonin reuptake inhibitor) (Patent Literature 1), and to possess a wide therapeutic spectrum in the treatment of central nervous system diseases (in particular, schizophrenia).